Age related macular degeneration (AMD) is the most frequent disease causing loss of vision in adults aged 50 and over. The formation of new blood vessels, subretinal neovascularization (SRNV), creates the visual impairment. Treatment is not possible in a large percentage of cases because SRNV often underlays the fovea and treatment would severely damage central vision, and, because the SRNV is often difficult to visualize and diagnose with conventional ophthalmic instrumentation.
One method of visualizing the fundus involves injecting the patient with a fluorescent dye agent which enters the blood vessels of the eye fundus and generates a fluorescence emission image of the vessels when excited by a particular wavelength of light. One such dye is Sodium Fluorescein which requires blue light to excite a fluorescent emission. However, since blue light does not deeply penetrate the retina, Sodium Fluorescein is only able to image vessels on or near the surface of the retina.
Indocyanine green (ICG) is a well known water soluble tricarbocyanine dye that binds almost 100% to protein in the blood and fluoresces in the near infrared which is capable of deeply penetrating the retina. Infrared ICG angiography highlights the choroidal vasculature with limited obstruction from retinal processes, including hemorrhages or leakages, or the retinal pigmented epithelium (RPE) layer overlaying the choroid. Furthermore, infrared ICG angiography allow immediate monitoring of areas of the retina that have been recently treated to confirm vessel closure.
However, due to its relatively low fluorescence intensity of only about 4%, relatively high intensity illumination from a conventional angiography light source is necessary to excite ICG to produce useful fluorescence emission images. This requires exposing the retina to high intensity broadband irradiation, of up to 200 mw/cm.sup.2, capable of damaging the retina.